Propagation Speed of γ-radiation (Rγ) in Air (a)

Transcription

1 1 Propagation Speed of γ-radiation (Rγ) in Air (a) Osmar F.S.L. Neto, (b)marcelo A.V. Macedo Jr., (c)josé T.P.D. Cavalcante, (c)henrique Saitovitch (a)fundação Técnico-Educacional Souza Marques Av. Ernani Cardoso, 335; Rio de Janeiro, RJ (b)instit. Federal de Educação, Ciência e Tecnologia do Rio de Janeiro; Rua Lucio Tavares, 1045; Rio de Janeiro, RJ (c)centro Brasileiro de Pesquisas Físicas CBPF/MCT; Rua Dr. Xavier Sigaud, 150; Rio de Janeiro, RJ - Brasil Abstract - propagation speed of 22Na-isotope γ-radiation 511 kev was measured in air, in different propagation distances variation of 0.10m; final results comparable to CODATA value. Key words: propagation speed in air, 22Na-isotope, electronic coincidence method 1) Introduction - The propagation speed (PS) of visible light -a short frequency range in the large frame of electromagnetic radiations (ER)- in air was measured, during the last hundred years (1), using a great deal of different methods, with high precision results being achieved. Presently, a conventionally accepted value is c= 299,792,458 m/s(2) (c reporting to the latin word celeritas: speed swiftness ). The value of ER's PS in a gas, air here included, is approximately the same when compared to such number in vacuum, because of the very small ER dispersion in gases. Therefore, as usual, we may admit that ER's PS in air and vacuum have the same value. Until present, such studies focusing propagation speeds, refractive indexes, dispersions were specially related to visible light, or to ER in wavelengths ranges close to it, and with transparent material media (MM). A first incursion in this subject dealing with Rγ was performed using an electronic coincidence counting measuring system (3). This method went on to be applied, with improvements concerning newer electronic modules and scintillation crystals for Rγ detection in order to get better time resolution measuring setups (MS)(4,5,6,7). To perform such measurements the availability of a Rγ source in which two Rγ are emitted simultaneously in opposite directions turns out to be essential to the feasibility of the experiment, as far as no reflection techniques could be used. Such suitable source, in all cases, was the positron emitter 22Na placed in a metal container in which the positrons are stopped and annihilated when reacting with the medium electrons, in such way originating -as it is very well established from momentum/energy conservation laws(8)- two Rγ, energy 511 kev each, both emitted simultaneously in opposite directions. Presently, the same method of measuring electronic coincidences related to 22Na Rγ emission was applied; as a temptative innovation step, the Rγ detection system time resolution was improved when compared to the until present equivalent equipment used. 2) Experimental The MS (Fig. 1; Fig. Photo 9) included two Rγ detectors, DET1 (photomultiplier XP-2020Q + BaF2 scintillator) and DET2 (photomultiplier XP CsF scintillator), each of them connected to an Fig. 1: Measuring Setup (MS) electronic fast-slow coincidence circuit [slow branch: amplifier (AMP), timing single channel (SCA), universal coincidence (COINC); fast branch: constant fraction timing discriminator (CFTD); time to pulse amplitude converter (TAC)]. Finally, the slow-fast coincidences were recorded on an analog digital converter/multi-channel (MCA). More detailed explanations about construction and performance of such an MS can be found elsewhere(9,10). A total number of seventeen experiments on air were performed and in all of them DET1 and DET2 were attached to a 2 m iron trail, on opposite sides of a ~ 5 μci/22na γ-emmiter. The experiments consisted in the measurements of the transit-time differences of the two oppositely emitted Rγ, as far as they appeared as coincidence spectra displayed in a Multi-Channel, according the different distances, each of them 0.10m appart from the other, DET2 assumed on the trail. Operationally, the seventeen spectra were divided in one set with five spectra and three sets with four spectra measured in such a way that the distance difference

2 2 between them, in each set, was 0.40m, a distance that avoided their interference upon the measurements due to MS time resolution. Finally, all the seventeen measured spectra of the four sets were intercalated computationally. All the so measured coincidence spectra were fitted with gaussian function, as founded x ², and the so obtained fitted parameters are in A in the ORIGIN software(11) y= y 0 exp [ ] 2 ² 2 Table II. As a first step of the experiments, it was measured the time calibration of the MS by using a Time Calibrator (TC) which produces two pulses with highly precise variable delays between their outputs; which, by their way, were directed to the TAC whose amplitude outputs is related to those delays. Finally, the average time calibration displayed by the MS was ns/ch, a result extracted from fitting (Figs. 2-3). Fig. 2 - Time-Calibrator Output Spectra Fig. 4 - DET2 Trail Positions: 0.0m-1.60m Fig. 5 - DET2 Trail Positions: 0.20m-1.40m Fig. 3 - Time-Calibration Line Experiment 1 (Exp. 1): in this experiment the five coincidence spectra were recorded 0.40 m each one apart from the other. The difference between both extreme spectra's fitted centers channels, corresponding to 1.60 m, was estimated: ch ch = ch; taking into account the time interval related to this difference, ns, the PS cair= 1.60 m/ ns= 300,675,705 m/s % of CODATA value. Experiment 2 (Exp. 2): in this experiment the four coincidence spectra were recorded 0,40 m each one apart from the other. The difference between both extreme spectra's fitted centers channels, corresponding to 1.20 m, was estimated: ch ch = ch; taking into account the time interval related to this difference, ns, the PS cair = 1.20 m/ ns = 299,054,390 m/s % of CODATA value.

3 3 Experiment 3 (Exp. 3): in this experiment the four coincidence spectra were recorded individually 0.40 m each one apart from the other. The difference between both extreme spectra's fitted centers channels, corresponding to 1.20 m, was estimated: ch ch = ch; taking into account the time interval related to this difference, 3, ns, the PS cair= 306,938,812 m/s % of CODATA value. Fig. 6 DET2 Trail Positions: 0.30m-1.50m Experiment 4 (Exp. 4): in this experiment the four coincidence measured spectra were recorded individually 0.40 m each one apart from the other. The difference between both extreme spectra's fitted centers channels, corresponding to 1.20 m, was estimated as: ch ch = ch; taking into account the time interval related to this difference, ns, the PS cair= 306,445,468 m/s % of CODATA value. Fig. 7 DET2 Trail Positions: 0.10m-1.30m Experiment 5 (Exp. 5): this experiment is a computationally composition of the previous four sets of experiments in the sense that their measured spectra were intercalatedly overlapped, totalizing a distance of 1.60m between first and last spectra, always according their positions in the abcissa/time coordinate. The difference between each spectra's fitted centers channels corresponds to 0.10 m. It would be expected that the mid-points fitted peaks would keep between them the same distances. But, as it can be seen in Fig. 8, this is not the case; the differences, even very small if Fig. 8 - Composition of Overlapped Spectra we consider the 0.10 m value in time, are probabely due to very small variations of the TC output pulses concerning their time-amplitude. The average of propagation speed of Rγ in air was estimated by the sum of the values and divided for sixteen intervals: the PS in air is 4,829, /16 cair= 301,817,077 m/s 0,67534 % larger than the CODATA value, a very good approach.

4 4 3) Concluding Remarks a) the electronic γ-γ coincidence method showed to be a valuable method to measure PS of electromagnetic radiation in air, even when measured in very short distances. b) in the above estimations it was taken into account only the fitted peak's centers of the coincidence spectra, neglecting any deviation concerned these values as done, for instance, by the fwhm of those spectra. c) the PS final results are closely related to the MS calibration conditions due to crystal scintillator's shapes and sizes, as well as detectors distances to emmiting source. d) possibility of measuring such PS in material media non-transparent to visible light, a topic that our Lab. is presently extending to plastics and metals. 4) References: (1) The Speed of Light - J.H. Rush; Scientific American, August (2) CODATA The Committee on Data for Science and Technology. (3) The velocity of γ-rays in Air - M.R. Cleland & P.S. Jastram; Physical Review 84/2(1951)271. (4) Measure of the Speed of Gamma-Rays as a Test of Performance of a Fast Timing Coincidence System - M. Lo Savio et al.; NIM in Physics Research A355(1995)537. (5) Speed of Pair Annihilation Gamma-Rays - Dept. Physics, Middlebury College; physics@middlebury.edu. (6) Speed of Light Measurements Using BaF2 Scintillation Detectors - L. Chow et al.; Eur. J. Physics 15(1994)49. (7) Measuring Speed of Light in Laboratory Using Na-22 Source and LaBr 3 Detector - A Thirunavukarasu; Tata Inst. Fund. Research, Mumbai; 29/03/2010. (8) Física Moderna para Iniciados, Interessados e Aficionados - I.S. Oliveira, Cap. IX; Ed. Livraria da Física, (9) Espectroscopia de Radiação-Gama - H. Saitovitch et al.; LCA/EXP/CBPF; Monografia/CBPF-MO-001/07; março/2007; attached references about instrumentation. (10) Radiation Detection and Measurements - Glenn F. Knoll; J. Wiley&Sons, eds (11) Table II (a): Fitted Parameters spectra channels fitted centers Xc (μ) fitted spectra centers amplitudes (H) fwhm (ch) ΔXc: [(n+1) n] (ch) ΔXc x calibration (ns) Exp , , , , , Exp , , , , , , , , Exp. 3 Exp , , , ,

5 5 Table II (b): PS Estimation on each 0.10m Adding Exp. 5 distance (m) ΔXc: [(n+1) n] (ch) ΔXc x calibration (ns) Propagation Speed (PS) of γ-radiation in Air 0.10m: = ,810,366 m/s 0.20m: = ,167,821 m/s 0.30m: = ,686,915 m/s 0.40m: = ,866,027 m/s 0.50m: = ,027,128 m/s 0.60m: = ,964,000 m/s 0.70m: = ,679,068 m/s 0.80m: = ,851,042 m/s 0.90m: = ,734,244 m/s 1.00m: = ,148,165 m/s 1.10m: = ,927,982 m/s 1.20m: = ,784,855 m/s 1.30m: = ,925,872 m/s 1.40m: = ,142,848 m/s 1.50m: = ,681,181 m/s 1.60m: = ,675,705 m/s Fig. 9 - MS photo